Novel procedure for starting the flash-extrusion of expandable resin compositions



Aug. 12, 1969 J, GILARm 3,461,193

NOVEL PROCEDURE FOR STARTING THE FLASH-EXTRUSION v OF EXPANDABLE RESINCOMPOSITIONS Filed Jan. 4, 1967 lNV ENT OR fUJL'KTZ/QHN 6/mk04 ATTORNEYUnited States Patent ice NOVEL PROCEDURE FOR STARTlNG THEFLASH-EXTRUSHON OF EXPANDABLE RESIN COMPOSITIONS Robert John Gilardi,Richmond, Va., assignor to E. I. du Pont de Nemours and Company,Wilmington, Del., a corporation of Delaware Filed Jan. 4, 1967, Ser. No.607,304 Int. Cl. 1329f 3/06 U.S. Cl. 26453 2 Claims ABSTRACT OF THEDESCLOSURE In flash extrusion of foams, sponges and plexifilamentarystrands a polymer solution at high temperature and pressure is extrudedinto a region of reduced pressure, resulting in flashing of the solventand solidification of the polymer. In starting up, inert gas underpressure is supplied to the enclosed path between the solution supplyvalve and the extrusion orifice to prevent premature flashing of thesolvent and solidification of the polymer.

BACKGROUND This invention relates to the flash-extrusion ofpolymer/liquid mixtures and more particularly to an improved procedurefor beginning flow of the mixtures through a die having at least oneflow-restricting extrusion orifice.

Flash-extrusion, as herein referred to, is a process wherein a polymer/liquid dispersion or solution at elevated temperature and pressurepasses through a flow-restricting orifice into a region of reducedpressure such that substantially all the liquid is vaporized almostinstantaneously and the polymer stream is simultaneously greatlyexpanded and cooled to solidification by the vaporization of the liquid.Products of so-defined flash-extrusion include closed-cell foams, foamswith interconnecting cells, polymer sponges, and fibrillatedplexifilamentary strands. These products are generally in the form offilaments, yarns, or thin sheets.

More specifically, flash-extrusion as defined herein deals with theextrusion of an expandable composition comprising a thermoplasticpolymer and a liquid, said liquid having an atmospheric boilingtemperature well below (i.e., at least C. below) the polymer meltingtemperature and being a non-solvent for the polymer at or below the saidboiling temperature. Before extrusion into a region of sharply reducedpressure, the expandable composition is at a temperature above that atwhich polymer freezes out and under a pressure at least suflicient tomaintain the liquid state. Said liquid is present at between about 98%by weight of the composition and a minimum amount sufiicient to sorapidly precipitate the polymer on adiabatic vaporization that molecularorientation is frozen into the solidfied structure. This minimum amountis ordinarily about by Weight of solution.

A typical and preferred flash-extrusion process is that disclosed byBlades and White in U.S. 3,227,784, the disclosure of which isincorporated herein by reference. The procedure of the presentinvention, however, is not limited to the process of the aforesaidpatent, particularly as regards its use only of crystalline polymers. Byway of illustrating further the preferred processes to which theprocedure of this invention is directed, reference is made to U.S.Patent No. 3,227,664 dealing with closed-cell ultramicrocellularstructures and to U.S. Patents Nos. 3,081,519 and 3,227,794 dealing withfibrillated plexifilamentary strands.

The viscosities of expandable compositions suitable for flash-extrusionare much lower than normally associated 3,461,193 Patented Aug. 12, 1969with, for example, customary foam extrusion. This characteristic resultsfrom the high concentration of liquid necessary to provide sufficientlyrapid polymer solidification to freeze into the solidified structuremolecular orientaton arising during extrusion and expansion of thepolymer. Consequently, high flow rates during extrusion and very narrowextrusion orifices are required in order to maintain an elevatedpressure at the entrance to each orifice and the guarantee a precipitouspresure-drop across each orifice. Suitable orifices have diameters fromabout 0.003 to about 0.050 inch (0.076 to 1.270 mm.) and preferablybetween about 0.005 and 0.030 inch (0.127 to 0.762 mm). In asheet-forming die the corresponding dimension is the gap-width. As isreadily apparent, such small openings can become plugged with onlyminute solid impurities.

In practical commercial flash-extrusion, an expandable composition isadmitted to each extrusion die through point of supply such as a valve.This valve is kept closed until the supply-line or supply-vessel forexpandable composition is full and at properly adjusted temperature andpressure. Ordinarily, for reasons which will be hereinafter discussed,there will exist between the point of supply and the point of extrusionof the composition an enclosed path of substantial volume. On openingthe valve to begin flash-extrusion, the volume of enclosed path betweenthe valve and the final extrusion orifice must first become filledbefore steady-state extrusion conditions can be established. This volumeis sometimes hereinafter designated the post-valve volume. Because thisflashextrusion is characterized by rapid flashing-off of liquid, i.e.,by polymer solidification within a small fraction of a second afterpassage through the extrusion-orifice, the first portion of expandablecomposition entering the postvalve volume ordinarily forms sufficientsolid polymer to plug the extrusion-orifices. This plugging of theorifices is aggravated by the necessity for incorporating within thepost-valve volume such devices as filters, pressure-detection probes,flow-distribution plates, and the like. As described in the aforesaidU.S. Patent No. 3,227,794, the post-valve volume preferably includes apressure-letdown, flow-restricting orifice upstream of the finalextrusionorifice when fibrillated plexifilamentary strands are to beproduced.

SUMMARY This invention provides a novel procedure for starting theflash-extrusion of expandable compositions without the disruptiveformation of solid polymer within the postvalve volume.

According to the procedure of this invention, upon start-up of theprocess inert gas under pressure is introduced into the enclosed pathbetween the point of supply and the point of extrusion of the expandablecomposition before the expandable composition is admitted to theenclosed path. Introduction of the gas is continued until the pressurein the enclosed path exceeds the autogenous pressure of the expandablecomposition but is no greater than the pressure applied to thecomposition in the supply. Thereafter, the expandable composition isadmitted into the enclosed path and is extruded in the customary way.Flashing of the liquid and solidification of the polymer before reachingthe point of extrusion are prevented by the cushion of pressurized inertgas in the enclosed path.

DRAWINGS FIGURE 1 is a schematic representation of a flashextrusionsystem to which the flow-start-up procedure of this invention applies.

FIGURE 2 is a schematic representation of a single extrusion positionindicating a supply-valve, an extrusion DESCRIPTION The generalizedapparatus to which the procedure of this invention applie is representedby FIGURE 1. A hot, pressurized, liquid-phase expandable composition iscontained in a supply-manifold 10. At least one extrusion-position 11(and ordinarily a plurality of them) is attached to supply-manifold 10.Each extrusion-position 11 has a supply-valve 13 for beginning orterminating the flow of expandable composition through the post-valvevolume to one or more extrusion orifices in each extrusion die 15.Pressure on the expandable composition in manifold is at least greatenough that, even after the pressure losses resulting from flow throughthe post-valve volume, the expandable composition is above itsautogenous pressure as it enters each exit-orifice in extrusion-dies 15.

As used herein, the term autogenous pressure denotes the minimumpressure on an expandable composition at a given temperature whichmaintains the liquid state and prevents the formation of an equilibriumvapor-phase.

When a single sharp pressure let-down across only the exit-orifice isall that is required in the flash-extrusion process, it is possible tomount that orifice directly in the wall of manifold 10. Valves 13 aretherefore eliminated by substitution of external closure devices, andthere is no post-valve volume. This alternative has numerousdisadvantages which dictate against it in practical commercialproduction. First, it is usually necessary to mount external processingapparatus close to the exitface of each die 15, and orifice closuredevices would interfere with placement and/or operation of saidapparatus. Secondly, without valves 13 at each extrusionposition 11, itis practically impossible to interrupt flow through one extrusion-die 15(e.g., to service it or clean it) without shutting down the wholesystem. Supplyvalves 13 also make possible changing the number ofoperating positions 11 without costly shut-downs. It is also desirableto interpose at least some transfer line between manifold 10 and dies 15in order that dies 15 can be arranged spatially with respect to oneanother somewhat independently of manifold limitations. The mostcompelling reason for creating a post-valve volume is, however, thatother functions are usually either desirable or necessary. Thus, it isnecessary for product uniformity to measure and control pressure on theexpandable composition just before it enters the final extrusion-orificebut after it has experienced all pressures reductions occurring in thepost-valve volume. Experience has shown it desirable to provide a filterfor the composition just before entering the extrusion-orifices in orderto remove any accidental impurities capable of plugging the orifices.Finally, a preferred method of forming fibrillated plexifilamentarystrands involves passing the expandable composition through a pressurelet-down zone between two orifices in series, which automaticallycreates at least some post-valve volume. While the existence of apost-valve volume is seen practically to be necessary, it should be keptas small as possible.

FIGURES 2 and 3 are schematic representations, partially incross-section, of single extrusion-positions 11 adapted toflash-extrusion as defined. Like parts in the figures are designated bythe same numerals. Supply-valve 13 is provided for beginning orterminating flow of expandable composition, which ultimately passesthrough exit-orifices 21 for expansion and solidification of thethermoplastic polymer. The enclosed volume of these systems betweensupply-valve 13 and exit-orifices 21 is the post-valve volume.

In the extrusion-position 11 of FIGURE 2, adapted to the production offibrillated plexifilamentary strands, a pressure let-down orifice 23 isprovided upstream of exit-orifice 21, a pressure let-down zone 22 beingformed therebetween. In continuous operation, pressure on the expandablecomposition both above and below let-down orifice 23 is above autogenouspressure; but above orifice 23 it is sufiiciently high to maintain asingle liquid phase while below it two liquid phases exist comprisingvery fine droplets of solvent-rich liquid uniformly dispersed in acontinuous polymer-rich phase. Regulation of pressure within pressurelet-down zone 22 is critical to the provision of suitable fibrillatedplexifilamentary strands, and a pressure sensor 24 is provided therein.

The extrusion-position 11 of FIGURE 3, while capable of producingplexifilamentary material, is especially adapted to the production offoamed strands. The only orifices provided are exit-orifices 21, andpressure upstream of these orifices 21 is both superautogenous and highenough to maintain a single liquid phase. Again, pressure upstream oforifices 21 is critical to properties of the foamed strand beingproduced, and a suitable pressure sensor 24 is provided.

In the apparatus of both FIGURES 2 and 3, a filter 25 is placed upstreamof the orifices 21 and 23. Filter 25 is ordinarily a screen-pack withmesh fine enough to prevent the passage of any particles capable ofplugging the orifices but with open area very large with respect toorifice-area so as to prevent large relative pressure drops acrossfilters 25. A pressure-gauge 26 may be provided to measure pressurewithin the post-valve vOlume upstream of the flow-restricting screensand orifices.

Also provided along the post-valve volume can be one or more inlets oroutlets for other functions. In the figures, this is represented by avalve-block 31 wherein, for example, valve 28 permits diverting the flowof expandable composition, valve 29 is an inlet for hot solvent used toclean the post-valve volume, and valve 30 admits pressurized inert gasthrough an injection-probe 32.

Finally, an automatic flow-control valve 27, responsive to a signalgenerated by pressure-sensor 24 is preferably provided along eachextrusion-position 11. It can be above, below, or combined withsupply-valve 13. In continuous production, wherein filters 25 may becomepartially fouled, automatic valve 27 serves to maintain criticalpressures at the entrance to exit-orifices 21. When only shortproduction runs designed for testing the effectiveness of start-upprocedures are made, or when only a single extrusion-position operatesfrom manifold 10, automatic-valve 27 can be dispensed with so thatpressure regulation in manifold 10 is responsive to pressure read bysensor 24.

Although not indicated in the figures, means for heating all surfacesdefining the post-valve volume are externally provided to preventcooling of the expandable composition during operation.

The foregoing has been intended to clarify typical continuousflash-extrusion processes useful in preparing expanded structures ofthermoplastic polymer and to describe suitable apparatus at eachextrusion-position 11. Before extrusion starts, however, hot pressurizedexpandable composition in manifold 10 is kept from the postvalve volumeby supply-valve 13, the post-valve volume being empty. On openingsupply-valve l3, expandable composition rushes into the post-valvevolume, solvent flashes off, solid polymer precipitates, and filters 25and orifices 21 and 23 become fouled and/or plugged.

In the procedure of this invention, inert gas at a pressure usuallyabout the same as that of the expandable composition above supply-valve13 is fed through a suitable injection probe 32 into the post-valvevolume until pressure-gauge 26 registers the required pressure,whereupon supply-valve 13 is opened and nitrogen flow is stopped. Theextrusion then proceeds smoothly with the formation of continuouslengths of foams, sponges, or plexifilamentary strands. Use of the inertgas results in successful start-up substantially 100% of the time.

The practice of the invention is uncomplicated and highly advantageous.No orifice-closures are needed. The inert gas, being of low heatcapacity, need not be preheated. It is readily available at the requiredpressures. Substantially none of it dissolves in the expandablecomposition, and polymer is insoluble in it. Consequently there is notendency for the gas to dilute the composition which could lead to theformation of fly, i.e. non-continuous webs of moist, tacky, fluffypolymer, upon extrusion.

An inert gas is any material which remains gaseous at the necessarypressures, which is substantially unreactive with both expandablecomposition and retaining walls, and which is substantially insoluble inthe heated composition. Such inert gases are well known. Of them,nitrogen is particularly preferred because it is inexpensive and readilyavailable at the required pressures. Both air and super-heated steam areoccasionally suitable inert gases. In some cases, however, oxygen in aircan promote excessive corrosion of apparatus; and, in production ofplexifilamentary strands, steam interferes with the necessaryelectrostatic charging of the strands.

Inert gas can be admitted anywhere within the postvalve volume, e.g.,through injection probe 32 via valved inlet 30. When the pressure in thepost-valve volume (gage 26) exceeds autogenous pressure for theexpandable composition, but is no greater than its applied pressure,valve 13 is opened and the flash-extrusion process commences. Apreferred probe 32 for injection of inert gas comprises a spring-loadedcheck valve, e.g., a poppet valve, which is opened by high differentialpressure but is automatically closed by the spring when the differencebetween inlet and outlet pressures becomes less than a predeterminedamount, e.g., 100 p.s.i. (7.0 kg./cm. Likewise effective is a three-wayvalve serving either as inlet 32 for inert gas or as supply-valve 13depending on which way it is switched. Otherwise, demanding manualmanipulation can be required to prevent flow of the composition into theinlet for inert gas.

The inlet 32 for inert gas is preferably upstream of allflow-restricting orifices (e.g., pressure let-down orifice 23) andespecially of final filter-screen 25. This location has the advantagesthat:

(l) accidental solid impurities in the gas stream are removed beforereaching orifices 21 or 23; v

(2) the gas expands on contact with the heated filter 25 to generatemore back-pressure and reduce the time necessary to reach requiredpressure in the post-valve volume;

(3) the flow-resistance of filter 25 reduces total gas-flow andsubsequent cooling of orifices 21 and 23.

On the other hand, it is frequently desirable to locate inlet 32 forinert gas as far downstream as is consistent with being upstream of theorifices and filter screens, this arrangement permitting gas-flow tocontinue until expandable composition has a minimum remaining pathbefore reaching exit-orifice 21. The pressure increment for injectedinert gas above autogenous pressure for expandable composition should begreat enough to maintain at least autogenous pressure on the compositionuntil it reaches exit-orifice 21. This required pressure increment canbe determined only by trial for each system. To obviate thisdetermination and also to prevent the possible initial formation ofnonstandard product, the maximum pressure exerted by inert gas withinthe post-valve volume is usually selected to be approximately equal to,but no greater than, the pressure on the supply of expandablecomposition. Frequently this state is achieved when the static pressureon the supply of inert gas is actually slightly greater than thepressure on the composition.

The start-up procedure of this invention is not intended to be limitedonly to flash-extrusion as specifically described hereinabove or in theexamples. It applies equally to the flash-extrusion of any expandablepolymeric composition wherein a volume between supply-valve andexitorifices exists, and in particular to the extrusion of dispersionsas well as solutions. The following examples, wherein all parts andpercentages are by weight, further illustrate the scope and advantagesof this invention.

Example I Fibrillated plexifilamentary strands were to be produced usingapparatus substantially as shown in FIG- URE 2. In place of automaticflow-control valve 27, a metering gear-pump was substituted. The uniformsolution in the supply-manifold was 12% linear polyethylene and 88%methylene chloride at about 209 C. and under about 860 p.s.i.g. (60.5kg./cm. gage). On starting the gear-pump and then opening supply-valve13, the

system immediately plugged. Apparently the methylene.

chloride flashed-off within the pump sufiiciently that thehigh-viscosity concentrated solution or molten polymer within it causedthe pump to stall.

A tank of pressurized nitrogen adjusted to deliver the gas at about 900p.s.i.g. (63.3 kg./cm. gage) was connected to the system via aneedle-valve 30 and 32. Before opening supply-valve 13, nitrogen wasadmitted to the system until pressure at the gear-pump discharge reachedabout 900 p.s.i.g. (63 kg./cm. gage), the gearpump motor was turned on,supply-valve 13 was opened, and then quickly and simultaneously thegear-pump was speeded up to deliver solution at about 900 p.s.i.g. (63kg./cm. gage) and at about 318 grams per 3 minutes, and the nitrogenvalve was closed. The extrusion of fibrillated plexifilamentary strandbegan and continued with no difficulty.

The above comparative results were duplicated in extrusions usingsolutions of 10 to 12% polymer at supply pressures ranging from 800 to1000 p.s.i.g. (56 to kg./cm. gage). Pressurizing the die-volume withinert gas prior to opening the supply-valve was shown to effectively andinextensively eliminate plugging associated with the beginning ofsolution flow.

Example II Apparatus substantially as described in connection withFIGURE 2, but arranged specifically as shown scrematically in FIGURE 4,was provided for the production of fibrillated plexifilamentary strandmaterial. In this arrangement the post-valve volume was that volume ofthe system between three-way valve 44 and exit-orifice 50. This systemdiffered from that illustrated by FIGURE 2 principally in the use ofthree-way valve 44. Initially, valve 44 was closed to hot polymersolution in manifold 40 under elevated pressure as indicated by gage 41.Simultaneously, it was open to flow of nitrogen under pressure asindicated by gage 43. Auxiliary valve 42 was provided to begin orterminate nitrogen flow. Also provided, but not shown, was piping forthe introduction of hot trichlorofiuoromethane, in place of thenitrogen, for subsequent cleaning of the post-valvevolume. On openingvalve 44 to permit passage of polymer solution from manifold 40, valve44 simultaneously terminated nitrogen flow. Thus, through the preferreduse of a three-way valve, valve-block 31 as indicated in FIGURE 2 waseliminated.

The uniform, single-liquid-phase, polymer solution in supply manifold 40contained 12.8 parts of linear polyethylene per 87.2 parts oftrichlorofluoromethane, was at about 185 C., and was under about 1790p.s.i.g. (126 kg./cm. gage) of pressure. Two filters were employed, theupstream filter 46 using 50 mesh screening and the other filter 47 using-mesh screening (both US. Sieve Series). Let-down orifice 48 was 0.026inch (0.66 mm.) in diameter and 0.025 inch (0.63 mm.) long. Exit-orifice50 was 0.020 inch (0.51 mm.) in diameter and 0.025 inch (0.63 mm.) long.A tunnel 51 co-axial with exit-orifice 50 was provided to direct theplexifilamentary strand against an oscillating baflie deflector 53 whichin turn directed the strand downward across the surface of chargingplate 54 thence onto a moving electrostatically charged collection belt(not shown). The tunnel was 0.125 inch (3.18 mm.) in diameter and 0.1875inch (4.76 mm.) long.

The purpose of let-down zone 49 was to drop pressure on thesingle-liquid-phase solution sufficiently that two liquid phases formed.At the solution concentration and temperature specified, pressurerequired for formation of two liquid phases was below about 1290p.s.i.g. (90.7 kg./cm. gage) but above the autogenous pressure ofapproximately 515 p.s.i.g. (36.2 kg./cm. gage). Pressure sensor 52within let-down zone 49 provided the signal by which automaticflow-control valve 45 maintained a preselected pressure within zone 49.

Solution-flow through the post-valve volume was started as follows:

(1) Automatic flow-control valve 45 was set at its 100% open manualposition.

(2) Nitrogen at 1780 p.s.i.g. (125 kg./cm. gage) was admitted viathree-way valve 44.

(3) As soon as pressure in let-down zone 49 reached 1500 p.s.i.g. (105kg./cm. gage), three-way valve 44 was turned to interrupt nitrogen-flowand begin solutionfiow. As solution followed nitrogen through thepostvalve volume, momentary pressure fluctuations occurred. Pressure inmanifold 40 decreased from 1790 p.s.i.g. (126 kg./cm. gage) to 1730p.s.i.g. (122 kg./cm. gage), and then returned to 1780 p.s.i.g. (125kg./cm. gage). Pressure within filter 46 decreased from a maximum of1745 p.s.i.g. (123 kg./cm. gage) to 1600 p.s.i.g. (112 kg./cm. gagePressure within let-down zone 49 decreased to a stable value of 1100p.s.i.g. (77 kg./Crn. gage).

(4) Automatic flow-control valve 45 was adjusted to control the let-downpressure in zone 49 at 950 p.s.i.g. (67 kg./cm. gage).

Solution-flow began and continued with no problems. No fine, tacky,polymer fluff formed; surfaces of oscillating deflector 53 and chargingplate 54 remained perfectly clean; and there was no disruption ofproduct laydown on the moving collection belt.

Successive successful start-ups of multiple spaced extrusion positions,as indicated in FIGURE 1, were made using substantially the sameconditions and procedures of this example. Wide, uniform, continuous,plexifilamentary, nonwoven polyethylene sheets were thereby produced.

Example III Apparatus similar to that represented by FIGURE 3 wasattached to a supply of expandable composition at conditions selected toproduce an ultramicrocellular foam.

The expandable compositon was a solution of polyethylene terephthalatein methylene chloride at a 60/40 (polymer/ solvent) ratio. Relativeviscosity of the polymer was 54.8 (relative viscosity is here defined asthe ratio of absolute viscosities at 25 C. of an 8.7% polymer solutionand its solvent, the solvent being composed of 70 parts of2,4,6-trichlorophenol in 100'parts of phenol). The uniform solutioncontinuously discharged from a mixer was collected in a variable-volumecylindrical accumulator closed at its top with a hydraulically operatedpiston. The apparatus was attached at the bottom of the accumulator.

Only one extrusion position 11 with a single orifice 21 was employed.Automatic-valve 27 was thus unnecessary and was not provided. Nor was avalve-block 31, as shown, provided. The die-assembly containingscreenpack 25 and an orifice-plate was fastened directly to thedownstream leg of ball-valve (supply-valve) 13, and suitable pressuresensing devices were provided along the length of the extrusion-position11. The single orifice 21 was 0.012 inch (0.305 mm.) in diameter with alength-todiameter (L/D) ratio of- 1.0. The screen-pack was a 200- meshscreen covered with a 20-mesh screen and supported from below with twolayers of 20-mesh screen (all U.S. Sieve Series). A thermocouple formeasuring solution temperature was mounted in the extrusion die.

Pressure on the-solution in the accumulator was 1600 p.s.i.g. (112.5kg./cm. gage). A nitrogen supply was adjusted to supply nitrogen atabout 1500 p.s.i.g. (105.5 kg./cm. gage). Nitrogen flow was begunthrough on opening upstream of screen-pack 25 to pressurize the systembelow supply-valve 13.

When nitrogen-flow had been established and the supply valve was opened,extrusion of the polymer-solution started perfectly and continued sountil the end of the supply of solution. As soon as flow was establishedthe pressure applied to solution in the accumulator was reduced to about850 p.s.i.g. (59.8 kg./cm. gage) to produce asolution pressure justupstream of the orifice of about 800 p.s.i.g. (56.2 kg./cm. gage). Thesolution temperature was about 214 C. The product was a continuousfilament of ultramicrocellular polyethylene terepthalate.

Example IV Another extrusion start-up was attempted using substantialythe same equipment, procedure and conditions as in Example III. Theorifice-plate in this case, however, had four spaced orifices 0.012 inch(0.305 mm.) in diameter (L/D=1.0). Upstream of the orifice-plate butdownstream of the screen-pack was interposed a metering plate with fourholes axially aligned with those in the orificeplate. Each of theseholes was 0.025 inch (0.635 mm.) in diameter with L/ D=5 .0. Theexpandable solution was polyethylene terephthalate and 35% methylenechloride. A valved nitrogen supply at 1800 p.s.i.g. (126.5 kg./cm. gage)was connected via a pressure-relief valve to the opening just downstreamof the supply-valve but well upstream of the screen-pack. Pressure onthe solution in the accumulator was also about 1800 p.s.i.g. (126.5kg./cm. gage), but a pressure-drop of about p.s.i.g. (7 kg/cm? gage) onnitrogen-flow through the pressure-relief valve was known to exist, thusguaranteeing that nitrogen pressure downstream of the supply-valve wouldnot exceed the upstream solution pressure. Solutign temperature abovethe supply-valve was about 215 2 0 C.

In quick succession, nitrogen flow was begun until pressure inside theflow system reached a maximum, the supply-valve was opened, and thenitrogen-valve was closed. Nitrogen issued for a few seconds from theorifices, followed immediately by four foamed filaments. After about 10minutes operation, three samples of prod-, uct issuing from each orificewere collected. Each was rendered fully inflated by brief immersion inliquid 1,1,2- trichloro-l,2,2-trifluoroethane followed by drying inambient air. Measured diameters for these samples were:

Average diameter Individual diameters Orifice N0. in. mm (in.)

I 054 l. 37 .055, .054, .053 2 055 l. 41 .056, .055, .055 3 055 1. 40.056, .054, .055 4 053 1.35 .053, .053, .053

The above results show excellent orifice-to-orifice uniformity andcompletely successful start-up. Solution temperature near theorifice-plate was about 215 C.

Example V I 9 I claim: 1. In a flash extrusion process wherein anexpandable composition comprising a thermoplastic polymer and a liquidat a temperature above that at which the polymer freezes out and under apressure above the autogenous pressure of the composition is fed from asupply and extruded into a region of reduced pressure causing the liquidto flash and the polymer to solidify, and wherein an enclosed path ofsubstantial volume exists between the point of supply and the point ofextrusion, the improvement which comprises, upon start-up of theprocess, introducing an inert gas into the enclosed path.

prior to admitting the expandable composition to the enclosed path untilthe pressure of the gas in the enclosed path is in excess of theautogenous pressure of the composition but no greater than the pressureapplied to the composition in the supply UNiTED STATES PATENTS 2,330,93210/1943 Taylor et a1. 264-85 XR 3,102,365 9/1963 Sneary et al 264-53 XR3,215,760 11/1965 Grace et a1 26485 XR 3,227,784 1/1966 Blades et al.264-53 3,389,198 6/1968 Taber 26488 XR PHILIP E. ANDERSON, PrimaryExaminer US, Cl. X.R.

